This invention relates to feedforward amplifiers used in cable communication systems and particularly to the construction of a unitized gain block for a feedforward amplifier.
Recently so called "feedforward" amplifiers have been utilized in cable communication systems because of their relatively distortion-free amplification. Distortion-free amplification allows cable companies to increase the spacing of the amplifiers in their communication lines with a concurrent decrease in the number of amplifiers used. Furthermore, feedforward amplifiers have a wide bandwidth and thus can amplify a large number of channels having widely disparate frequencies. Broadband amplification is necessary as more and more cable systems increase the number of channels available to their customers.
Feedforward amplifiers are able to provide relatively distortion-free amplification by extracting from the amplified signal a signal component representative of the distortion introduced by the amplifier, phase inverting that component, and then recombining the phase inverted distortion component with the amplified signal. The phase inverted distortion component cancels the distortion component in the amplified signal, leaving a relatively distortion-free amplified signal for transmission along the cable. To avoid further distortion, the signals must coincide precisely in time when they are recombined. To ensure such coincidence, delay means are provided to delay the signals and thereby compensate for time delays which arise when the signals pass through the various electronic components in the amplifier.
In U.S. Pat. No. 4,472,725 (Blumenkranz), the disclosure of which is hereby incorporated by reference as if fully set forth herein, electronic delay lines particularly suitable for use in a feedforward amplifier in a cable communication system are disclosed. The delay lines disclosed in this patent provide the precise time delay necessary for efficient and accurate operation of the feedforward amplifier. However, merely providing precisely matched delay lines does not eliminate all possible sources of phase variation between the main and the error signals. Ideally, the main and the error signals should have paths through the directional couplers, delay means and amplifiers which are identical in length. However to date, the circuit topology of the circuit board and particularly, the main and error amplifiers, has not permitted the main and error signal paths to be precisely matched.
Additionally, prior feedforward amplifiers have utilized delay lines which are mounted to the main circuit board as separate subassemblies. The use of separate subassemblies, in addition to making it more difficult to provide matched signal paths through the amplifier, increases the size, cost and difficulty of assembly. Furthermore, if the size of conventional feedforward amplifiers is reduced by conventional means, amplifier heat transfer problems may arise. The present invention is directed to providing a unitized gain block for a feedforward amplifier having matched signals paths, minimum size and maximum heat transfer.
It is an object of this invention to provide a feedforward amplifier in which the paths of the main and error signals through the directional couplers, delay lines and amplifiers are identical.
It is another object of this invention to provide a feedforward amplifier in which the delay lines, directional couplers and attenuator circuitry are integral with, and disposed on, the main circuit board.
It is another object of this invention to provide a feedforward amplifier which minimizes the size of the circuitry while maximizing the heat transfer from the main and error amplifiers.
It is another object of this invention to provide a feedforward amplifier that enhances serviceability by facilitating replacement of the main and error amplifiers.
Still other objects of the invention will become apparent upon a reading of the detailed specification to follow.
To these ends, a unitized gain block for a feedforward amplifier is provided. The directional couplers, delay lines and attenuator circuitry are all disposed integrally on a single circuit board in a symmetrical arrangement having matched signal paths. The main and error amplifiers are mounted separately from the main circuit board via releasable connectors. The main and error amplifiers are arranged on their substrates as mirror images of each other to provide symmetrical signal paths. The amplifiers and the circuit board are enclosed within a compact metal housing which provides heat sinking for the amplifiers, yet permits ready removal of the amplifiers for servicing.